Support Node Expressions as constraint parameters

[HolgerKnublauch]

The Core spec currently has a red TODO item to allow node expressions in most of the constraint parameters such as sh:datatype or sh:minCount. All of these values currently need to be constants and cannot be computed depending on the context like node expressions could.

This ticket here is to apply this generalization to the Core document and to add test cases and a couple of examples from the SPARQL document.